A new study indicates many car buyers may be overburdening themselves with payments they can't really afford by choosing more costly models.

The median income in the metro areas of 25 major cities was $57,391 last year, Interest.com, an offshoot of Bankrate.com, found. That means that following its guidelines, families should limit their new car payment to $410 a month, which is enough to buy a new car for up to $20,806.

Just one problem: The average price of a new car last year was $30,550 — indicating that many purchasers may have bought more car than they can afford. Buyers would need to shell out $601 a month on a typical loan to pay it off.

"What this research indicates, more than anything, is that a lot of Americans are spending too much money on their cars," said Mike Sante, managing editor of Interest.com, in a statement. "Car costs are one of the most controllable parts of a household's budget."

It also indicates that new cars are out of reach for families that pay attention to their spending. Most midsize family sedans are priced higher than $20,000 when critical options are counted in. And in some cities with lower median incomes, families may be hard-pressed to buy the cheapest economy car, much less one that best suits their needs.

In only one city of the 25 checked did the study find cars were affordable: metro Washington, D.C., where the median income is $86,680.

What's affordable? Interest.com says follow the "20/4/10" rule: Pay a down payment of at least 20%, keep the loan to no more than four years and make sure the loan principal, interest and insurance don't exceed 10% of a household's gross income.

Using that formula, here's what Interest.com says are the maximum monthly car loan payments based on average incomes in these cities: